The traditional approach for cooling turbine blades and nozzles is to extract high pressure cooling air from a source, for example, by extracting air from the intermediate and last stages of the turbine compressor. A series of internal flow passages are typically used to achieve the desired mass flow objectives for cooling the turbine blades. In contrast, external piping is used to supply air to the nozzles with air film cooling typically being used. However, the diverted coolant air does not receive energy directly from the combustors of the turbine and represents a parasitic loss to turbine output and degrades overall performance.
In advanced gas turbine designs, it has been recognized that the temperature of the hot gas flowing past the turbine components could be higher than the melting temperature of the metal. It is therefore necessary to establish a cooling scheme to protect the hot gas path components during operation. Using closed circuit steam to cool gas turbine nozzles (stator vanes) has been demonstrated to be a preferred cooling media, particularly for combined-cycle plants. See, for example, U.S. Pat. No. 5,253,976 of common assignee herewith. Because steam has a higher heat capacity than the combustion gas, it is inefficient to allow the coolant steam to mix with the hot gas stream. Consequently, it is desirable to maintain cooling steam inside the hot gas path components in a closed circuit. A complication arises, however, that the coolant supply pressure level is usually much higher than that of the hot gas stream. Thin walls, desirable for cooling purposes, may not be sufficient to withstand the pressure difference without incurring excessive mechanical stresses. That is, the necessary thickness to maintain structural integrity would drastically increase the magnitude of the required cooling flow. Thus, it has been found desirable to provide nozzle vane segments which can be cooled while operating under high thermal fluxes and high pressures.
Moreover, because of the various and multiple piping arrangements necessary to steam cool the non-rotating components in the hot gas path of the turbine, it is not practical to cool certain regions of those components with steam supplied in a closed circuit. For example, the relatively thin structure of the trailing edges of the nozzle vanes effectively precludes steam cooling of those edges.